The invention relates to proportional solenoid-controlled valves and more specifically to a normally closed spool type solenoid valve which as it begins to open changes it opening or orifice size in proportion to the amount of current supplied to the solenoid.
Normally closed solenoid valves, such as shown in U.S. Pat. No. 3,737,141, are the type which pull a spring-biased poppet off a seat as the valve opens. Before the poppet opens, the solenoid force must overcome a pressure-induced force which holds the poppet closed. The amount of this force is dependent upon the pressure differential across the poppet and the area of the poppet seat. This pressure-induced force decreases as the valve opens, while the force produced by the solenoid increases as the valve opens. When the air gap between the pole piece and the armature is the largest, the force the solenoid generates is the weakest, which is converse to the requirements of a conventional poppet valve.